The majority of the solar cells currently produced are based upon crystalline silicon.
A conventional solar cell with a p-type (p-doped) silicon base has an n-type (n-doped) emitter in the form of an n-type diffusion layer on its front-side. Such conventional silicon solar cell structure uses a negative electrode to contact the front-side or sun side of the cell, and a positive electrode on the back-side. It is well known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate electron-hole pairs in that body. The potential difference that exists at a p-n junction, causes holes and electrons to move across the junction in opposite directions, thereby giving rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal contacts which are electrically conductive. Typically, the front-side metallization is in the form of a so-called H pattern, i.e. in the form of a silver grid cathode comprising thin parallel finger lines (collector lines) and busbars intersecting the finger lines at right angle, whereas the back-side metallization is an aluminum anode in electric connection with silver or silver/aluminum busbars or tabs. The photoelectric current is collected from the front-side busbars and the back-side busbars or tabs.
MWT silicon solar cells are examples of silicon solar cells having another cell design than the conventional silicon solar cells described in the preceding paragraph. MWT silicon solar cells are well-known to the skilled person (cf. for example, F. Clement et al., “Industrially feasible multi-crystalline metal wrap through (MWT) silicon solar cells exceeding 16% efficiency”, Solar Energy Materials & Solar Cells 93 (2009), pages 1051-1055). MWT silicon solar cells represent a special type of silicon solar cells; they are back contact cells allowing for less front-side shadowing than standard silicon solar cells. The p-type silicon wafers of MWT silicon solar cells are provided with small holes forming vias between the front- and the back-side of the cell. MWT silicon solar cells have an n-type emitter extending over the entire front-side and the inside of the holes. The n-type emitter is covered with a dielectric passivation layer which serves as an ARC (antireflective coating) layer, as is conventional for silicon solar cells. Whereas the n-type emitter extends not only over the entire front-side but also over the inside of the holes, the dielectric passivation layer does not and leaves out the inside of the holes and, optionally, also a narrow rim around the front-edges of the holes. The inside of the holes and, if present, the narrow rim around the front-edges of the holes, i.e. the n-type diffusion layer not covered with the dielectric passivation layer, is provided with a metallization either in the form of a conductive metal layer (open hole) or in the form of a conductive metal plug (hole filled with conductive metal). The metallizations of the holes are typically applied from one or two conductive metal pastes and fired. To avoid misunderstandings, if two different conductive metal pastes are used, they are not applied so as to form a double-layer metallization; rather, one conductive metal paste is applied to the holes from the front-side thereof and the other from the back-side. The metallizations of the holes serve as emitter contacts and form cathodic back contacts of the MWT silicon solar cell. In addition, the front-side of the MWT silicon solar cell is provided with a front-side metallization in the form of thin conductive metal collector lines which are arranged in a pattern typical for MWT silicon solar cells, for-example, in a grid-or web-like pattern or as thin parallel finger lines. The term “pattern typical for MWT silicon solar cells” means that the terminals of the collector lines overlap with the metallizations of the holes and are thus electrically connected therewith. The collector lines are applied from a conductive metal paste having fire-through capability. After drying the collector lines so applied they are fired through the front-side dielectric passivation layer thus making contact with the front surface of the silicon substrate.